In thickness measurement of geometrically extensive objects, a number of problems arises, in particular when, such as in industrially-preprocessed sheets, the thickness of the objects to be tested can vary in a wide range, such as from 0.01 mm to several centimeters. If the geometrical extension in relation to the thickness of the objects is large or, in one direction, even approximately infinite, such as in materials, such as foils, paper or sheet metals wound on rollers, the problem exists that the thickness of a large area of the object has to be determined with a single measurement for keeping the overall measurement period at an acceptable level. Additionally, the problem arises that, in particular with materials produced in the form of webs, the demand for the tolerance of the thickness of the web is typically particularly high. For example, in automotive engineering, the sheet metal strengths are partly specified with extremely low tolerances, since maintaining the sheet metal strength is significant for the crash behavior of the finished vehicles. Additionally, when controlling sheet metal thicknesses, it also has to be considered that sheet metals are typically produced with high velocity in rolling mills, so that a large sheet metal area has to be tested per time unit. Among others, industrial rolling machines are used that can generate sheet metals with a width of up to 3 meters. Additionally, the sheet metal consists of a material that is opaque for conventional optical radiation, which additionally impedes measurement of sheet metal strength. Tactile methods that can determine the thickness of the sheet metal in a spatially resolved manner by directly contacting the surface of the same, can hardly be used for such a purpose, since the same would have to mechanically detect a plurality of measurement points, which would increase the effort and thus, the cost for the quality control significantly. In industrial production methods, sheet metals are frequently generated so fast that the same exit at the output of a rolling apparatus with such high speeds that these sheet metals vibrate in a direction perpendicular to the surface. In such cases, the usage of tactile methods is, in principle, not possible. Similar considerations apply, apart from sheet metals, to a plurality of other planar materials, such as foils, paper, glasses or similar elements, which necessitate specific material strength control with high accuracy.
Due to the problems outlined above, monitoring the wall strength during production is extremely expensive, with sheet metals it is generally performed radiometrically, i.e. by using radio-active sources or X-ray tubes for generating X-radiation and detectors sensitive to radio-active rays or X-rays, respectively. Thereby, the material to be tested is screened with X-radiation or gamma radiation and the wall strength of the screened material is determined by the ray attenuation caused by absorption of the radiation in the material to be tested. Therefore, the ray intensity or the original ray intensity, respectively, has to be known and the ray intensity remaining after screening the material has to be detected by appropriate detectors. Radiation-sensitive detectors are generally very expensive apparatuses. Currently, for example, normally counting tubes are used, which means detector tubes filled with gas and provided with high voltage, since the same are relatively long-term stable and show little drift (for example, temperature-induced). When monitoring the production of broad sheet metals, sometimes up to 100 of such detectors, and possibly several X-ray sources, have to be used for obtaining the necessitated spatial resolution or sensitivity, respectively, of the thickness measurement across the whole width of the sheet metals that are up to 3 meters wide. Here, realistically obtainable measurement accuracies are in the range of 0.1% of the wall thickness, which means approximately 10 μm in sheet metals of 10 mm. An obvious disadvantage is the high costs induced by such a measuring apparatus. For example, a high-voltage channel of a high-voltage supply and a read-out or evaluation channel, respectively, of a signal processing electronic has to be provided for every counting tube.
A further disadvantage is that the obtainable measurement accuracy is determined by the statistics of X-radiation (Poisson statistics). Thus, the signal to noise ratio is determined by the square root of the detected X-ray quanta. With the given available measurement time, the spatial resolution or the thickness sensitivity, respectively, is limited. Although basically the measurement accuracy can be increased by a longer measurement period or integration time, respectively, this is not arbitrarily possible in industrial scale, since the material coming out of a production line has to be tested within a limited period of time. The basically possible activity increase of the used X-ray sources also increases the risk in a radiation accident and can, thus, only contribute in a limited manner to increasing the measurement speed or the obtainable measurement accuracy, respectively.
Additionally, for the usage in connection with extensive materials, such as sheet metal rolls, the X-ray method is only partly suitable, since the typical commercial detectors (counting tubes) have an extension of merely several centimeters, so that, as has already been mentioned above, a plurality of such detectors has to be used. Additionally, the obtainable spatial resolution is heavily limited by the finite extension of the counting tubes, since an individual counting tube can only detect the existence of a gamma quantum in the area covered by the same, wherein a further differentiation of the location of the gamma quantum within the counting tube is not possible.
The conventional X-ray methods have the disadvantage that they can obtain only a limited spatial resolution and that they use detectors, whose acquisition and operation is extremely complex and cost-intensive.
The international application WO 91/08439 relates to generating sectional views or sectional representations, respectively, of extensive three-dimensional objects via light-slit measuring methods. A light measuring strip is projected onto the surface of the object via at least two light sources. The projected light measuring strip is imaged via several cameras, wherein the images are supplied to an evaluation means. When the geometry of the arrangement is known, the evaluation means can reconstruct the outline of the projected light measuring strip.
U.S. Pat. No. 4,564,296 describes a method for determining the thickness of a plate, wherein an optical test pattern is projected onto the surface of the plate to be measured. For determining the distance between two optics arranged on different sides of the object to be measured, a test pattern is projected onto the surface of the object. The distance is determined by shifting or varying the optic until the projected test pattern on the surface of the object reaches maximum sharpness. Further, an area with a reference thickness exists on the object carrier mechanically holding the object to be measured.